The present invention relates to a mobile radio telephone system having a plurality of speech channels and at least one common control channel, and more particularly to a system for detecting any malfunction in a transmitter/receiver for the common control channel thereof.
Currently available mobile systems of radio telephone communication include the so-called cellular system, wherein the service area is divided into a plurality of small zones, a mobile base station being installed in each zone, and communication is achieved between a mobile subscriber and a fixed subscriber or between mobile subscribers over one of plural speech channels assigned to the mobile base station concerned. There are assigned in this system to each mobile base station for common use by the plurality of mobile subscriber stations in the zone covered by the mobile base station, in addition to a plurality of speech channels, a paging channel for controlling incoming calls to the mobile subscriber stations and an access channel for controlling outgoing calls therefrom. One example of a mobile radio telephone communication system having such a channel arrangement is described in a paper titled "800 MHz Band Land Mobile Telephone Control System" published in "REVIEW OF THE ELECTRICAL COMMUNICATION LABORATORIES", pp. 1172-1190, Vol. 25, Nos. 11-12 (Nippon Telegraph & Telephone Public Corporation, November-December 1977).
In a mobile radio telephone system for achieving communication over multiple speech channels, it is intended to reduce the time required for making call connections and to efficiently use the radio frequencies assigned thereto by the specialized call connection control of the paging and access channels. However, because the paging and access channels are commonly used in a given radio zone, in the event of a problem in the radio transmitter/receivers or on the wire line for the paging and access channels, the mobile subscriber stations present in the zone covered by the troubled mobile base station will become unable either to transmit or to receive calls, affecting the functioning of the whole system.
Therefore, any trouble on the wire line or in the radio transmitter/receivers for the paging and access channels has to be quickly detected and made known to the mobile control center controlling the mobile base stations. The detection of problems on the paging and access channel of a mobile base station is achieved, for example, by constantly monitoring the outputs of the transmitter/receivers. In this way, trouble detection on the paging channel alone can be achieved with comparative ease, because a control signal is initiated on the paging channel from the mobile control center. This system, however, cannot easily detect trouble on the access channel of the mobile base station, due to the random transmission of call signals on the access channel from the mobile subscriber station. Thus, when there is no output from the access channel transmitter and receiver, it is impossible to determine whether the absence of an output is attributable to a problem on the access channel transmitter and receiver or to the absence of a call from the mobile subscriber station.
There has been developed a system in which a test transceiver (TTR) is installed in each mobile base station. A maintenance man, by remote control from the mobile control center, actuates each TTR to send a test transmission signal in response to a report of trouble from a mobile subscriber or in accordance with a periodic maintenance program to detect any trouble on the access channel. For further details on such a system, reference is made to a paper titled "Supervisory and Control Equipment", published in "NEC RESEARCH & DEVELOPMENT", No. 57, pp. 99-108 (April 1980). Although it is a useful system capable of detecting problems on the access channel of the mobile base station, it has a disadvantage in that, if the intervals of maintenance by the maintenance man are too long, it may take a long time for him to detect a problem after it arises or, if the maintenance intervals are reduced to quicken trouble detection, the detecting process will hinder communication by other mobile subscriber stations sending out ordinary call signals.
Another approach adapted to quickly and automatically detect such problems on the access channel receiver is proposed in the pending U.S. patent application Ser. No. 313,406, filed on Oct. 21, 1981, entitled "Method of Detecting Troubles in a Receiver for a Common Control Channel of a Mobile Radio Telephone Communication System and a System Therefor," and assigned to the assignee of this application.